1. Field of the Invention
The present invention relates generally to the installation, maintenance, and testing of fluid transmission lines, and in particular, to method and apparatus for freezing a slug of water in a large diameter, high-pressure pipeline for hydrostatic testing and repair or modification purposes.
2. Description of the Prior Art
Large diameter pipelines have been used for many years to transport crude oil, natural gas, gasoline, jet fuel and other petrochemical products over great distances. These pipeline systems gather the petroleum products from the field and transport them to refineries and processing plants. The finished products from the refineries and processing plants are than transported over long distances to market.
These pipelines must be operated at high pressures in order to economically transmit large volumes of fluid. The high operating pressures represent a significant percentage of the maximum pressure which the pipe will withstand before rupture or leak, even if in perfect condition. Because of the nature of the petroleum products, rupture of a pipeline, or even relatively small leaks, can seriously damage the ecology and endanger human life. New pipelines may leak or rupture for a number of reasons, including faulty manufacture of the pipe, faulty weld joints, or damage to the pipe during transportation or installation. Pipelines which have been in use for some period of time are also subject to leak or rupture due to corrosion.
Because of the serious consequences and relatively high likelihood of ruptures and leaks, rigorous hydrostatic testing is employed to insure the integrity of each new pipeline installation. These hydrostatic testing procedures are exceedingly expensive and represent a very large proportion of the total cost of laying the pipeline. The hydrostatic testing of existing pipelines is even more expensive because the pipeline must be taken out of service for a substantial period of time.
Because of the substantial expense involved, methods for installing and testing new pipelines, or for testing, repairing or modifying existing pipelines are continuously being improved. One such improved method is disclosed in U.S. Pat. No. 3,827,282 issued to B. D. Brister. According to that disclosure, a large diameter pipeline designed to transport fluid products over great distances at high pressures can be tested at pressures equal to the minimum rated yield strength of the pipe by filling a test section of the line with water, freezing a slug of the water by the external application of very low temperatures to form an ice plug within the section, then raising the pressure of the water against the frozen water to the test pressure, while also sustaining the ice plug, and monitoring a lower pressure on the back side of the ice plug to detect a faulty ice plug. A combination of pressure cylinders and chill coils filled with a cryogenic cooling fluid such as liquid nitrogen are utilized to form the ice plug by conduction transfer of thermal energy from the slug of water in the test section through the walls of the pipeline and through the walls of the pressure cylinders and the chill coils. There remains considerable interest in the general improvement of the methods and apparatus which utilize a cryogenic cooling fluid to form the ice plug. In particular, because of increased labor costs and loss of product revenue associated with downtime, a further reduction in the length of time required to establish an ice plug is desired.